Aluminum is a metal which offers a good compromise between electrical conductivity, mechanical strength, weight and cost. As such, the use of aluminum wire or cable as an electrical conductor has increased significantly in recent times. There are many possible applications where aluminum wire or cable could be used only if certain physical and mechanical properties are achieved. One of the most important applications is an overhead transmission conductor.
Steel reinforced aluminum cable (ACSR) or aluminum conductor steel supported (ACSS) for use as overhead transmission conductors have been developed for decades. For example, U.S. Pat. No. 3,813,481 discloses a steel supported aluminum overhead conductor (SSAC). According to this patent, conventional 61% IACS (International Annealed Copper Standard) aluminum rod is drawn by conventional means to wire form in a drawing step, then the drawn wire is fully annealed. This drawn, fully annealed wire is soft and easily subject to damage and, thus, must be handled carefully in a subsequent stranding step. That is, since the wire is extremely soft (“dead soft”), the surface is easily scratched or damaged; such scratches are an important cause of arcing and corona in the finished overhead transmission conductor cable. Therefore, special precautionary steps must be performed during the stranding process. These precautionary steps include applying a liquid lubricant to the surface of the fully annealed aluminum wires, reducing the back-tension on the aluminum wires passing through the stranding machine, reducing the operating speed of the stranding machine, modifying the wire guides to minimize scuffing (which can cause scratches), enlarging the closure dies which press the annealed stranded wires against the steel core, and reducing the pressure of the closing dies.
As an attempt to solve the problems associated with the above patent, U.S. Pat. No. 5,554,826 discloses a method of producing an improved overhead transmission conductor. First of all, 99.8% (or greater) purity aluminum is selected to maximize the conductivity in the finished product. The aluminum is preferably continuously cast and rolled normally to form a rolled rod product. The aluminum rod product is then fully annealed by conventional methods at an elevated temperature for a time period sufficient to assure recrystallization resulting in a reduction of the tensile strength to approximately 9.0 kilopounds (thousands of pounds) per square inch (ksi). The annealed rod is drawn to the desired size, which introduces strain hardening of a strength in the range of 20.0 ksi. Then, a stranding operation forms the aluminum conductor wires into at least one layer having a spiral twist, or lay, over the stranded steel cable which forms the core. As a result of hardening occurring before and during the drawing and stranding processes, the aluminum components of the cable are not at the desired “0” temper or “dead soft” condition following stranding (as required by the ASTM B233-1350-O specification). The overhead transmission conductor is therefore subjected to a stress-relieving/annealing heat treatment to produce a dead soft condition in the aluminum components.
The conventional processes as discussed above embrace many problems and disadvantages in terms of the efficiency and cost of the processes. For example, the use of high purity aluminum is very expensive, as are full annealing treatments carried out before or possibly after the standing process.
There is, therefore, a need to overcome some or all such prior art problems and provide a new technology for producing an aluminum overhead transmission conductor in a cost effective manner.